Hydrophone development has undergone a long and continuous evolution. Many different configurations have varying degrees of sensitivities, bandwidths, etc. Ferroelectric materials have been used for some time and appear to be satisfactory for most purposes. Optical hydrophones recently have demonstrated that some of the problems normally associated with electrical conductors such as excessive power drain, bulk, cross talk, etc., can be avoided.
The Opto-Acoustic Hydrophone disclosed in U.S. Pat. No. 3,903,497 by Morton Stimler et al., concerns itself with the conversion of acoustical signals to corresponding modulated optical signals. This hydrophone transmits the converted signals over a fiber optic cable to a remote location. Acoustic signals impinge on a piezoelectric crystal which amplifies the signals to drive a light emitting diode and feed the signals to a demodulator for conversion to electromagnetic signals and ultimate transmission. This approach does eliminate the long electrical conductors, however, the transducing of the acoustic signals to the light signals and subsequent demodulation of the light signals for retransmission must necessarily degrade the signals validity.
Frank W. Cuomo's "Acousto-Optic Underwater Detector" is described in U.S. Pat. No. 3,831,137 and uses bifurcated bundles of optical fibers. These bundles are arranged in such a way that a light source irradiates an acoustically displaceable reflector which reflects the light back to a detector. It is alleged that the intensity of the reflected light provides an indication of the frequency and amplitude of the impinging acoustical signal.
Two more recent approaches proposed to perform remote, passive acoustic sensing are shown schematically in FIG. 1 and FIG. 2 of the drawings. Both devices rely on the interference of two light beams to determine the amplitude and frequency and impinging acoustic signal. In both methods of operation, the light output of a laser is divided by a beamsplitter into two beams which are launched into separate single-mode optical fibers. One fiber serves as a reference beam path and the other fiber transmits the signal beam to an acoustic sensor.
In the method of operation of the apparatus of FIG. 1, the signal beam is coupled out of the fiber and launched at an angle .theta. between two mirrors of high reflectance. One mirror is fixed while the other one is free to move in response to an incident acoustic signal. The movable mirror will be displaced by an amount A which is proportional to the acoustic pressure P of the incident wave. This mirror displacement will produce a phase change in the signal beam of .DELTA..theta.=(2.pi./.lambda.) (AN/cos .theta.) where .lambda. is the optical wavelength and N is the number of refections between the mirrors. After the beam exits the mirrors, it is coupled back into another section of signal-mode fiber and transmitted back to the point where the signal is to be observed. Here it is interfered with the reference beam resulting in optical intensity which is given by I(t)=I.sub.o cos.sup.2 (.pi.A(t)N/.lambda. cos .theta.). Thus, by measuring the amplitude and frequency of I(t), the acoustic signal amplitude and frequency can be determined.
Looking to the optical hydrophone scheme shown in FIG. 2, the acoustic sensor is a coiled, single-mode fiber of length L that suffers an index of refraction change when an acoustic pressure wave is incident on the fiber. This results in a phase change of the signal beam given by ##EQU1## where P is the pressure of the acoustic signal and .delta.n/.delta.p is a constant at a given acoustic frequency. When the signal and reference beams are recombined, the resulting intensity is given by ##EQU2## and the acoustic amplitude and frequency can be determined from measurement of I(t).
The disadvantages of these last two devices are similar. Both use separate signal and reference beam fiber paths which can result in relative phase shifts occurring between the two beams which are not due to signal phase shifts. The phase shifts instead are caused by differences in the acoustic/mechanical environments of the two fibers. Furthermore, in the device of FIG. 1, input/output coupling optics are required at the movable sensor. This increases the complexity of the device and makes it very difficult to fabricate.
Thus, there is a continuing need in the state-of-the-art for a highly accurate optical hydrophone which advantageously includes an accurate optical resonator and does not introduce error signals due to having separate reference and signal beam paths.